Depending on the age and full model number the problem is either a dirty flame sensor rod or a bad printed circuit card.. 95% chance that it is either one of those.
Tell me a lot about you experience working on such things, so that I can address these issues with you most effectively.
If you want extensive coaching
Please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to be taken from straight on, in very good light.
You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here. ,Include a picture of the thermostat wire connections to the printed circuit board in the same area.
If you have an error code showing, tell me what the code read ou tis on the wiring diagram or a sticker next to the diagram please.
If there is no paper clip-icon, look for the '+attach files'link at left of your send button.
I can mark it up with trouble shooting instructions.